Cooling the tube – Engineering heat out of the Underground

On a hot summers day a trip on the tube to get home can be a dreaded experience, with already hot trains overflowing with sweaty people. But how did we get to a situation where tube trains are stiflingly hot, and what’s being done about it?

A talk given by London Underground’s Head of Station Systems Engineering, Sharon Duffy, looked into the challenges the tube faces in keeping cool during the summer.

The “tube” can be split into two types of service — the tube proper which runs through tube tunnels, and the older sub-surface lines which are just below street level (pedants get very upset when sub-surface lines are called tube trains).

The older sub-surface tunnels were built for steam trains, so had loads of big holes in the ground included in the design to deal with removing smoke, and they are also much larger than tube tunnels. This has allowed the London Underground to fit air-conditioning units to its new fleet of sub-surface trains (S Stock), and as anyone who uses them appreciates, it’s a boon on hot days to get onto a cool train.

The heat from the air conditioning units is easily vented away when the trains are underground thanks to the pre-existing steam train ventilation.

The deep tube tunnels

However, it’s the deep level tube tunnels that cause the biggest problems for both passengers suffering the heat, and London Underground in getting rid of it.

The deep level tunnels suffer a number of problems which are individually a nuisance, but collectively add up to show why there is such a huge problem cooling the Underground.

One of the biggest problems is a side-effect of what made it possible to dig the deep level tunnels in the first place — namely the very solid and nice to tunnel through London Clay which sits under the city.

In fact, when the early tube tunnels were dug, they were so cool down there that the cool tube was seen as a respite from the summer heat on the surface. Why suffer on a bus in the heat when there’s a cool tube to take instead, said the marketing men.

So why is the Bakerloo line, once the coolest place to be, now a mobile sauna?

While that heavy thick clay is lovely to tunnel through, it is also a heat insulator.

Over the years, the heat from the trains soaked into the clay to the point where it can no longer absorb any more heat. Tunnels that were a mere 14 degrees Celsius in the 1900s can now have air temperatures as high as 30 degrees Celsius on parts of the tube network.

Where does the heat come from?

Well, the passengers aren’t the problem. All those hot sweaty bodies represent roughly 2 percent of the heat in the tunnels.

Climate change is also not much of a problem. It’s an impact, but the tunnel temperatures are not much affected by what’s happening up on the surface. During a heat wave in 2006, as the surface temperatures jumped around, the tunnels were pretty much constant.

About 21% of the heat in the tunnels comes from the movement of the trains themselves, from aerodynamic drag and other frictional losses. The motor engines account for 15%, the electrical and auxiliary systems are the remaining 12%.

About half the heat in the tunnels though comes from just one source –from the trains slowing down — the conversion of movement into heat by applying the brakes.

So it can be seen that cutting the heat from applying the brakes is where the biggest win would be, and indeed, the use of regenerative braking now converts about half the heat loss back into electricity. However, that can only work where trains are accelerating and braking at the same time, on the same electricity sub-station loop.

Experiments have been underway to improve that by use of an inverting substation, supplied by Alstom, which can send unused power from braking trains back into the national grid.

Removing the heat

Anyone who has stood on the platforms will know that as the trains approach, there’s a blast of wind. This is intentional, as the trains act as air pistons in the small tunnels, and the effect of pushing air ahead and sucking air from behind soaks away about 11 percent of the heat in the tunnels.

So when you curse as your paper flaps in the wind, think about the cooling benefits as well.

Mechanical ventilation removes about 10 percent of the heat — that’s the big ventilation shafts that line the tunnels.

The older tunnels weren’t built with a lot of ventilation, as it wasn’t thought to be necessary — after all it’s difficult to argue that tunnels will get hot when standing in a tunnel that’s cool enough that you need to wear a jumper.

By the time the Victoria line came along, the engineers were very well aware of the problem and it was built with considerably more ventilation shafts than older tunnels would have been supplied with.

Although it varies depending on location, in general cooler air is sucked down through the stations, and then ventilation shafts in the tube tunnels sucks out the hot air.

In two locations, they’ve added water chillers to the intake to further cool the air down. Some of you might have noticed the shockingly cold air flow on the eastbound platform at St Paul’s tube station.

Air is taken in through an old lift shaft, cooled down, then pumped down to the platforms. A similar design was recently installed between Blackhorse Road and Walthamstow Central on the Victoria line.

Ventilation isn’t just about cooling though. On newer networks, such as Crossrail, they also act as smoke control systems should the worst happen in a tunnel. During a fire, the shafts can in places reverse flow, to blow smoke in a preferred direction. The aim being that people walking down a tunnel walk towards a fan blowing fresh air down the tunnel, while the smoke is sucked away behind them.

Over the past few years, 14 of the Victoria line shafts have been upgraded, and 50 fans across the network have had their airflow doubled, with 10 out-of-action fans brought back into use.

Despite that, fully 79 percent of the heat in the tunnels is left to soak into the surrounding clay, which is already at or near its limit thanks to decades of absorbing heat.

The difficulty of adding more ventilation is the lack of space above ground to put new ventilation shafts. This is always going to be a problem for the older tube tunnels except on rare occasions when a surface development takes place at just the right location and agreements can be made to include a shaft down to the Underground.

It’s not just the cost of adding the new shafts and the running costs of all the electricity, but ventilation shafts also need sound attenuators to reduce the noise levels, both at the surface, but also in the tunnels so that people aren’t deafened by the noise.

People tend to be wary of having a new ventilation shaft in their neighbourhood, even though the aim is to keep the volume level to that equivalent to background city noise.

Even if new shafts are installed, at the moment they represent just 10% of the heat removed from the tunnels, so you can imagine how many extra shafts would be needed to remove a meaningful amount. So much land above the tunnels would be needed that you might as well just have a surface railway.

Other ideas

There are other ideas. You’ve probably seen the blue fans in some stations, and while technically they don’t do anything to remove heat (in fact, at an imperceptible level, they raise it), the air flow movement over human bodies helps evaporate sweat and that cools our own bodies.

Cooling the tube is as much about comfort as it is about actual lowering of temperature, so the evaporative effect caused by the fans makes us feel more comfortable, even if there’s no net temperature benefit.

Cooling the tube is also about safety, and one of the key measures used by London Underground is how long people can be stuck in a tunnel on a broken down train before suffering from the heat.

In 2001, more than 600 people needed heat shock treatment after being evacuated from three trains that were stuck on the Victoria line. So one way of reaching that safety measure is to stop trains breaking down. Indeed, it’s now almost unheard of for people to be evacuated through tunnels on foot. Reducing journey times and cutting delays means less time spent on a hot train, and that is indirectly a way of making the journey if not more pleasant, as least, less of an ordeal.

Elsewhere, they’ve been using cool ground water to cool some of the stations. An experiment at Victoria station was the first, as water from the River Tyburn was used to cool the air in the station. This was only an experiment, but at Green Park, a permanent version was installed in 2012.

There, five boreholes were drilled deep down into the ground, and here cool water is sucked up to the surface, used to cool a separate water supply which is pumped down to cool the platforms, and the warmed ground water is also pumped back into the ground.

The ground water isn’t directly used to cool the tunnels, as it needed to be kept in an isolated system to prevent any accidental contamination. Over the next 60 years though, they expect the ground water in the area to warm by a couple of degrees.

This is also a bit of an issue, as heat has to go somewhere, and generally at the moment it is dumped outside, mainly vented to the atmosphere. Much better would be to reuse the heat so that someone else doesn’t need to make their own heat.

An experiment in Islington is trying that very thing using heat from the tube tunnels to warm up a municipal heating service provided to a housing estate. The advantage of this scheme is that it can remove heat in winter when it’s needed above ground. It may seem mildly annoying that surface users don’t want heat in summer when you’d think the tunnels are at their most oppressive, but in fact removing heat in winter helps during the summer.

If the clay surrounding the tunnel can be cooled in winter, it has more capacity to absorb heat in the summer.

As it happens, at this particular trial, the fans can also be reversed so that during the summer months, they can suck cool night time air down into the tunnels as well.

So that’s one use for the heat, another is to turn the heat back into electricity. This is already happening to a degree with the regenerative braking, but heat in the tunnels may be removable through pipes, and then used in Trigeneration plants to generate electricity.

This would be not just useful for cooling the tunnels, but as the generators need to be local to the heat source, it also meets a Mayoral proposal for more local electricity generation in London to reduce risks of power outages from National Grid problems.

Trigeneration also has the curious ability to use heat to fuel absorption refrigeration systems, so providing cool water, which in turn can be pumped down to stations to cool them.

The issue is whether such schemes not just work, but are also cost-effective.

Something which has been built into Crossrail station designs, and where possible may be retrofitted to some London Underground stations is under platform air systems.

Knowing that more than half the heat in the tunnels comes from the brakes and motors, blowing cool air onto them when they are in the stations, or sucking away hot air, will help reduce the heat dumped into the tunnels when the trains leave the stations.

The warm air is them removed from the station by ventilation systems.

Ice cubes

And of course, there’s the ice cubes experiment.

Most of the tube tunnels have above ground sections, so a hybrid idea is to use air conditioning in the trains when above ground, and while above ground to cool a block of “phase change media”, or water to you and me, into an ice pack. When underground, the heat that would be dumped in the tunnels is absorbed by the ice-pack until it has returned to water.

Whether this can be viable is still being looked at, bearing in mind that they already struggle to fit air conditioning units into tube trains, finding space for the ice blocks is going to be even more of a headache. And not to forget that the extra weight means more energy needed to drive the trains, driving up running costs.

The future

As can be seen, even with all the planned upgrades, it’s never going to be viable to remove all of the heat from the tunnels, so the aim going forward is to stop the heat getting in there in the first place.

The specifications for the New Tube for London not only include squeezing some sort of air-conditioning units into the carriages, but much more importantly, they need to consume a lot less energy in the first place.

Lighter trains, with the ability to coast unpowered for longer with less energy consumed means less heat dumped into the tunnels.

The future of the cooling the tube project will be judged not so much by how they cool the hot tunnels, but by how they stop tunnels becoming hot in the first place.

29 comments on “Cooling the tube – Engineering heat out of the Underground”

It took LU at least 4 years to re-invent, for themselves, a “Not Invented Here” idea, that was put to them by several people ( guess how I know?). This was, of course, to use the now-surplus London Artesian Basin water as a cooling fluid for heat-exchange in deeper stations, & then pump it to the surface, where it could be flushed away, or used for street-cleaning. But, of course, they had to devise their own methods. I am not sure, even now, if they are using the “free” artesian water or not.

The Green Park borehole scheme does that, but it’s a vastly more complex operation than you suggest, as firstly they can’t just drain the water away it has to be returned to the basin, as LU is not the only user of the water, and there were some quite challenging issues to do with avoiding water contamination, acid lining of the boreholes etc.

They had to comply with a thick pile of environmental regulations just to carry out what was at the time still just an expensive experiment.

Thank you for this article! I enjoy learning technical information about situations we encounter on a daily basis. Could you please advise how I can find out which Underground lines are Tube and which are subsurface? And do you have an emailed newsletter I can sign-up for?

Great article and it avoids the usual much of the usual disingenuousness that TfL resorts to when “explaining” why buses can’t be cooled atc. (“Because we didn’t want to pay for it.” Now bugger off)

Some nitpicking – The statement “However, that can only work where trains are accelerating and braking at the same time, on the same electricity sub-station loop.” is understandable (to me) but needs explanation. I suspect this means that the power supply, being unique to the Tube, must remain balanced, and you can’t just shove a lot of extra energy back into it without providing somewhere for it to go. An inverter would change DC to AC and permit sending power to the grid – why not batteries as are becoming more and more common on the same grid? Most solar installations (DC generation) now have large battery banks that store energy and also even out demand. These don’t have to be in the Tube – just connected to the power supply.

As you say “The older tunnels weren’t built with a lot of ventilation” but there are newer sections… that (I’m betting) also were not built without adequate ventilation. And just how aggressive has LU been in identifying sites (like brownfield) where extra ventilation shafts could be quickly provided? Yeah, right. They’re not even bright enough to order TWO sets of parts for escalators or lifts when they refurbish them.

I’m fascinated by the concept of 30° clay under London. Since the mean temperature of London must be something like 10° (12.25°, roughly) you may have hit upon a huge source of “free” energy. Keep in mind many spaces need to be heated in London 10 months of the year. This would also appear to stretch some laws of thermodynamics (the heat wants to escape – the larger the differential the more so) and I suspect again this might be a “peak” reading used to make the case of “just how hard it is for us” compared to those temperate zones in Singapore (or even NYC).

Oh – I got a kick out of “Anyone who has stood on the platforms will know that as the trains approach, there’s a blast of wind. This is intentional”. Yes – I noticed that many train-in-tunnel systems failed to design that in, with the trains creating a vacuum in front of them. Another British triumph 🙂

Excellent article but I agree with Steve P on electrical regenerative braking – not a 10/10 explanation there as it requires a bit of thought (and a Physics degree) to see you don’t mean one train accelerating and braking at the same time, but having two or more trains in the same section: one or more accelerating and needing juice, and one slowing down and putting current into the system.

But what to do with that electrical energy if the energy produced by the decelerating train exceeds that needed to the accelerating one? One answer might be an on-board flywheel system such as in the Parry people move used in the West Midlands.

By the way tube trains have had a form of regenerative braking for a long time: as a train approaches many stations it goes up an incline to slow it down. Here the train’s kinetic energy is being turned into gravitational potential energy (GPE). When the train leaves the station, it goes downhill and the GPE is converted back into kinetic energy (KE).

Heat is produced from the brakes as the conversion of kinetic energy into GPE is usually far from sufficient to stop the train and so friction-based pads acting on discs bring it to a halt.

Perhaps the answer might be to lower the lines before a station, increasing the amount of GPE gained so that it just balances the KE an average train has just before the slope. (Tongue fully in cheek…)

Most (well all) substations currently only have transformer rectifiers that, as the name suggests, takes HVAC then transforms it down from high voltage and then rectifies to DC for connection to the conductor rails. You’d be surprised at the number of substations on the underground and much like the whole network are of various sizes, shapes and locations where installing additional equipment is easier said than done (even negating the cost of it). There is an inverter trial at Cloudsley Road that is currently ongoing where issues are being ironed out before potential expansion.

Traditionally there are many DC track sections, usually between substations, many reasons for this, some historic, reliability was also a concern, an issue in a certain section of track, for example, by a train with an earth fault wouldn’t bring the rest of the line to a standstill. Technology has moved on, power equipment is less sensitive to voltage fluctuations from the extra mini generators and with the greater use of regenerative braking, dc sections are being enlarged, with the victoria line in particular being almost one section now Brixton to Walthamstow.

This is, as discussed above, to ensure that there is a receptive load for a train with regenerative braking, otherwise normal friction braking is used. Extensive computer modelling is being used more frequently to determine future load requirements for increased train service and timetabling to try and make as much use of this as possible. Some of the older stocks, e.g. bakerloo 72TS has a primitive form of regenerative braking, rheostatic, where it is completely wasted through large resistor banks…which of course just adds to the heat problem!

Similarly to substations, there are a larger than you might think number of vent shafts dotted around the network and in very surprising locations hidden in plain sight. These mostly are there for providing ventilation rather than cooling but more and more are being upgraded to provide cooling as well as ventilation.

Like the scheme at City Road discussed in the presentation, there is a lot of interest in using the waste heat from the tube for district heating and can only see more schemes popping up in the future in our age of renewable energy.

I think it means that the clay is now at thermal equilibrium with the tubes. I.e., the clay surrounding the tubes, which 100 years ago was at 14 degrees, has over time warmed up to the same temperature as the tubes, which means that the clay isn’t going to get any warmer, but that it also can’t make the tubes any cooler. To put it another way, the conveniently low temperature of the clay turned out to be a non-renewable resource. (At least in the short run; stop the trains for 100 years and the clay would probably cool back down to where it was).

I do hope though that someone serious has looked into the possibility that raising the temperature of the clay by that much might affect its physical properties. That could be bad!

Great article. Thanks for this but I dont understand a bit of the explanation.

There is a direct correlation with a mildly warm day and it becoming hot on the deep lines – there is a cummulative effect even on the deep lines which go outside so should release their heat – not sure how this matches the claim that 2% of the heat is from passengers? Trains are not braking more, etc on warmer days. Surface heat will not penetrate the ground to that depth that quickly so it must be generated internally? The tube heats up within a day so people must definitely be more than 2% of the effect?

Similarly the Jubilee line has zero improved performance on the new extension where the ground wont have ‘reached capacity’ so how does this tally up with the explanation around insulation?

EXACTLY! The deep tube stations are about 10-20’C warmer today than a month ago. I am sweating like a pig in just a shirt. Whereas same journey in winter on the (deep) platform I’m not sweating in a full coat get-up.

So I kinda disbelieve all of the above based on empirical observations of my sweat!

Has anyone else noticed the correlation between the extreme summer weather conditions and the heat on the tube? All the mechanical and electrical systems don’t know it’s summer! My car hasn’t moved all day yesterday but the temperature on the gauge was registering 32 degrees. Cars and trains are fundamentally constructed of similar material so why wouldn’t the summer weather make the trains super-hot? The trains carry all the summer heat into the tunnels and discharge some of it. Please think about it! It may surprise you that the numerous temperature readings I have taken indicate that the trains gain much more heat in the summer when on the surface than they do underground. These tests could easily and cheaply be replicated and verified. Overheating wasn’t a problem in the winter – you probably kept your coat on in the tube then? Perhaps not surprising when the annual temperature range varies from say minus 5 to 35 degrees!http://stories.scienceinpublic.com.au/2016/warshippaint/ This would be part of my suggested solution.

Eric. I am not talking about ambient air, but train skin being super heated by solar. Obviously air temperatures figure in this as well. LU constantly talk about improved ventilation but even ventilation in my home does not address the fabrics heat for some time. Ventilation will improve the experience, because you won’t feel so “gagged” – but it is not cooling as such.

How about putting heat pumps into the basements of the buildings above the tunnels with small pipes going down around the tunnels or to large flat tunnel wall radiators. The buildings can then use the heat for their own heating and warm water.

The easiest way of preventing heat generation would be to automate the trains properly.

At the moment, there is constant acceleration and braking, both of which generate heat.

Even the simplest automated system could avoid much of this – knowing where the next signal is and its current state would allow a computer to determine the ideal acceleration / braking profile with which to approach it, thus minimising energy use and heat generation, while greatly improving passenger comfort.

The fact that drivers on the Waterloo & City line – the world’s least complex line – subject its passengers to constant acceleration and declaration between its only two stops stands as testament to the utter failure of TfL to confront its trade unionist drivers.

What a great article, and definitely one that will resonate with a lot of people suffering this week’s weather! David, your point is one of great significance – there does seem to be a lot of kinetic energy being thrown around and it’s frustrating to see trains working so hard between small distances.

Good article. In the meantime I wish trains could be open all the way along, as they are on the newer sub-surface trains (and have been in Paris for decades), which hugely improves air-flow when they’re moving, as well as making more space for passengers, and improving safety. At the moment all we have is those silly little windows marked “Lower window for ventilation” which have not changed since the 1940s!

There are also unnecessary glass screens which create pockets of hot air within a carriage. Just removing those, or replacing with poles or slotted screens, would improve air-flow at almost no cost.

I recall suggesting the use of groundwater to Dave Wetzel 20 years ago & how the passage connecting to the Tyburn was always cool. Remember phase change for water liquid to gas eats up 2.5 million calories per gallon – why people flock to parks on hot days

Already figured out ways to deliver emergency cooling for trains and NBFL with quick & basic solution & bigger scheme to reduce temp & humidity in tunnels so that heat can be more bearable as well as some reduction

Every article I read on the subject tells us when the tube gets hottest and when you should not travel without a bottle of water and I keep hoping that someone will get the clue as to why. I have been sharing my theory recently with passengers and underground staff and they have all said it makes perfect sense. I am hoping I can get the LU design staff to look at some different and simple monitoring to check my theory out.

Fascinating text and great way not only to learn about London´s situation but also to understand the difference between the Tube and Berlin´s U-Bahn. I´m also impressed by the expertise of some of the readers who left their comments here. Well done!

In terms of using the regenerative braking more frequently wouldn’t a simpler method to install batteries that can absorb the excess electricity generated and then feed it back in when other trains are accelerating thereby balancing the load and reducing overall electricity usage? If this removed a significant amount of the heat generated by friction from braking then would not making the trains more aerodynamic not also further reduce aerodynamic drag created heat and the push of air would not be needed as much?

I had an idea for cooling the tube, based on the idea that for much of the year, outside temperature are very low, and on some days well below freezing. Also that many off-peak trains are nearly empty. Passengers would be directed into certain carriages on these trains. When overground, outside of the tunnels, the doors would be opened to ‘scoop’ up cold air.Then when at the hottest points underground the doors would be opened in the tunnels to ‘deliver’ the cool air. Over many thousands of trains and many years, the cumulative effect of this practice might reduce underground temperatures. I wonder if anyone can do some calculations to see if this plan would make any difference.